Most metal recovery processes use aqueous solution extraction. Unfortunately, the aqueous extraction solutions usually contain toxic, caustic and harsh chemicals, such as cyanide, hydrochloric acid, sulfuric acid that form water soluble complexes with metals.
Within the past fifty years, cyanide has remained one of the most common chemical compounds used in the processes for recovery of metals from ore. Cyanide has the great ability to form water soluble complexes with metals. It achieves this via the fact that the cyanide molecule has what is called a triple bond between the carbon and nitrogen atoms. This special bond makes the cyanide molecule very reactive and it easily forms cyano-metallic complexes such as potassium gold cyanide (KAu (CN)2), sodium zinc cyanide (Na2Zn(CN)4) and sodium copper cyanide (Na3Cu(CN)4).
Most people know of cyanide as a dangerous, lethal chemical, and, without a doubt it can be dangerous if it is not treated appropriately. Humans can be subjected to the effects of cyanide via ingestion, inhalation, or absorption through the skin. In essence, upon intake of the cyanide, cellular asphyxiation occurs via binding of the iron in the cytochrome C oxidase enzyme. With the iron bound up by the cyanide, the human body cannot property utilize oxygen, the individual actually suffocates from oxygen starvation. Cyanide levels in the tailings (discharged waste) pond should be reduced to 50 milligrams per liter or less to avoid wildlife mortalities. Cyanide levels above 100 milligrams/liter can cause bird and other wildlife mortalities.
Hydrogen cyanide (HCN) is converted to calcium or sodium cyanide for mining purposes and shipped to the mine site, usually in briquette form. The solid cyanide briquettes are added to a tanked mixture of finely crushed ore and water and let stir for a period of many hours. The solid cyanide dissolves in the water portion of the mixture, attacks or leaches the metal in the ore, and forms the water soluble complex. Thus, the metal is extracted from a solid state (in the ore) to a liquid state (in the solution).
FIG. 1 is a schematic illustration of a traditional batch process using an aqueous cyanide solution to leach metals from metal-containing ores.
After a metal has been put in solution or liquefied, it is subjected to electrowinning, also called electro-refining or electroextraction. Electrowinning is the electrodeposition of metals from their ores onto a plate or wire mesh; this is an important technique that allows purification of a non-ferrous metal in an economical and straightforward step.
Various patents claim improvements in metal recovery and electrorefining processes developed since 1865 when a commercial process for electrolytic copper refining was patented by James Elkington.
U.S. Pat. No. 5,232,490 to Bender et al. uses an oxidation/reduction process for recovery of precious metals, such as silver and gold, from manganese dioxide ores, sulfidic ores and carbonaceous materials. The process involves leaching the ore with a leach liquor comprising an acid, such as hydrochloric acid (HCl) and sulfuric acid in the presence of a reductant to dissolve the precious metals. Recovery of the dissolved precious metals in the fluid can be by electrolysis.
U.S. Pat. No. 5,205,858 to Manke describes a precious metals recovery process using the standard cyanide-extraction technique together with carbon adsorption to facilitate the recovery.
U.S. Pat. Publ. No. 2003/0039605 A1 to Ramsay discloses a process for recovering precious metals from fine carbon bearing residual amounts of precious metals. The process involves incinerating carbon followed by a method for separating the precious metals from carbon ash; separation could include cyanidation, gravity concentration, smelting, electrowinning and solvent extraction.
U.S. Pat. No. 6,972,107 B2 to Marsden et al. describes a system for direct electrowinning of copper from a leach solution of a copper-containing ore, concentrate, or other copper-bearing material without the use of copper solvent extraction techniques or apparatus.
FIG. 1 shows a prior art batch process for recovering valuable minerals, such as, gold using an aqueous cyanide solution 4 to leach gold from an ore 1 that is crushed and/or ground 2 before it is conveyed to a filtration bed 3 consisting of a vault of pelletized fly ash (ground metal ore) containing gold. The cyanide solution 4 extracts the gold from the ore into the solution from which the valuable minerals (gold) are removed 5. The mineral extract is concentrated and refined for purification 6 and a toxic cyanide waste 7 is left for disposal and further handling to prevent damage to the environment. This process has a metal recovery rate between approximately 32% to approximately 37%, with many negative environmental and health impacts due to the use of cyanide and caustic materials.
The use of cyanide in processing also generates significant amounts of cyanide by-products that take time to degrade, notably cyanate and thiocyanate, and metal complexes of cyanide. The exact toxicities, residence time, and impacts on aquatic organisms are still poorly understood. It is common for regulatory agencies to omit monitoring for these compounds in the discharges from mines, partially because there is so little known about them.
None of the prior art processes for recovery of metals from ore use non-caustic compositions to extract metal from its ore. All of the processes have complicated, hazardous, costly processing steps. Many of the processes have disastrous consequences to the environment where the processes are performed. There is a need for a lixiviant that is non-irritant or non-deleterious to humans and the environment. There is also a need for an environmentally friendly, efficient process for recovering metals from ore that also conserves natural resources. The present invention fulfills the needs not met by the prior art and provides an improvement in the metal extraction efficiency of a commonly owned U.S. patent application Ser. No. 11/765,868 filed on Jun. 20, 2007.